Regulatory Matters - Schedule of Regulatory Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Detail of regulatory assets
Current	$ 197,507	$ 166,902
Non-Current	1,329,446	1,342,941
Pension
Detail of regulatory assets
Current	0	0
Non-Current	703,460	733,351
Retired power plant costs
Detail of regulatory assets
Current	28,182	28,182
Non-Current	146,076	167,164
Income taxes — allowance for funds used during construction (AFUDC) equity
Detail of regulatory assets
Current	6,457	6,457
Non-Current	154,269	151,467
Deferred fuel and purchased power — mark-to-market (Note 7)
Detail of regulatory assets
Current	41,643	31,728
Non-Current	27,305	23,768
Deferred property taxes
Detail of regulatory assets
Current	8,569	8,569
Non-Current	60,338	66,356
Deferred fuel and purchased power
Detail of regulatory assets
Current	59,474	37,164
Non-Current	0	0
SCR deferral
Detail of regulatory assets
Current	0	0
Non-Current	45,296	23,276
Four Corners cost deferral
Detail of regulatory assets
Current	8,077	8,077
Non-Current	34,171	40,228
Deferred compensation
Detail of regulatory assets
Current	0	0
Non-Current	37,589	36,523
Lost fixed cost recovery
Detail of regulatory assets
Current	25,775	32,435
Non-Current	0	0
Income taxes — investment tax credit basis adjustment
Detail of regulatory assets
Current	1,079	1,079
Non-Current	24,555	25,522
Ocotillo deferral
Detail of regulatory assets
Current	0	0
Non-Current	23,643	0
Palo Verde VIEs (Note 6)
Detail of regulatory assets
Current	0	0
Non-Current	20,480	20,015
Coal reclamation
Detail of regulatory assets
Current	1,546	1,546
Non-Current	18,821	15,607
Loss on reacquired debt
Detail of regulatory assets
Current	1,637	1,637
Non-Current	12,441	13,668
Mead-Phoenix transmission line CIAC
Detail of regulatory assets
Current	332	332
Non-Current	9,795	10,044
TCA balancing account
Detail of regulatory assets
Current	5,016	3,860
Non-Current	2,721	772
Tax expense of Medicare subsidy
Detail of regulatory assets
Current	1,235	1,235
Non-Current	5,073	6,176
AG-1 deferral
Detail of regulatory assets
Current	2,787	2,654
Non-Current	3,413	5,819
Tax expense adjustor mechanism
Detail of regulatory assets
Current	2,916	0
Non-Current	0	0
Other
Detail of regulatory assets
Current	2,782	1,947
Non-Current	$ 0	$ 3,185